1.Field of the Invention
The present invention relates to optical wavelength division multiplexers, and more particularly, to a multiplexer module for use with optical fibers, which is encapsulated in a small but rugged package.
2. Description of the Prior Art
Heretofore fiber optic wavelength division multiplexers were provided as assemblies of discrete components interconnected with optical fibers. FIG. 1 illustrates a typical prior art configuration wherein a wavelength division multiplexer 2 is formed by a dichroic filter 4 disposed between two graded index lenses 6 and 8. The dichroic filter 4 is designed to block and reflect light having a wavelength 11 and to pass light having a wavelength 1.sub.2. The graded index lenses 6 and 8 are formed with a quarter pitch, so that light received at an outermost surfaces is expanded to illuminate a large area of the dichroic filter 4.
An optical fiber 10 is attached to one end of the graded index lens 6 at a predetermined distance from an axis 12 of the graded index lenses 6 and 8. An optical fiber 14 for transmission purposes is attached to the graded index lens 6 at a point on a side of axis 12 opposite from the point of attachment of optical fiber 10, and at a predetermined distance from the axis equal to the predetermined distance that optical fiber 10 is from axis 12. An optical fiber 16 is attached to graded index lens 8 at a point spaced from axis 12 by a distance equal to the predetermined distance that fiber 10 is spaced from axis 12.
An electro/optical conversion element 18 includes an optical fiber pigtail 20 for connection to an optical fiber and electrical leads 22 for connection to electronic circuitry. Element 18 could be either a light source or a light detector. As a light source, element 18 would comprise either a light-emitting diode or a semiconductor laser. The pigtail 20 is attached to the optical fiber 10 by either fusion splicing, as at 24, or through the use of an optical fiber connector 24. A second electro/optical conversion element 26 may comprise either an optical source or an optical detector, having attached thereto an optical fiber pigtail 28 for connection to an optical fiber, and electronic leads 30 for connection to an electrical system. The optical fiber pigtail 28 is connected to optical fiber 16 either by fusion splicing, as at 32, or through the use of an optical fiber connector 32.
The element 18 is selected to emit or respond to light having a wavelength 1.sub.1, while the element 26 emits or is responsive to light having a wavelength 1.sub.2. The optical fiber 14 carries light of both wavelengths 1.sub.1 and 1.sub.2, either unidirectionally or bidirectionally.
The fiber optic wavelength division multiplexer assembly shown in FIG. 1 can be adapted to operate either unidirectionally or bidirectionally. In the case of a unidirectional operation, the assembly is designed as either a transmitter or a receiver. As a transmitter, light of both wavelengths 1.sub.1 and 1.sub.2 is transmitted on optical fiber 14. In the case of a receiver, light of both wavelengths 1.sub.1 and 1.sub.2 is received on optical fiber 14. In the case of a unidirectional transmitter, both elements 18 and 26 are light sources providing light to the optical pigtails 20 and 28. Light of wavelength 1.sub.1 is injected into the graded index lens 6 by optical fiber 10 at a point a predetermined distance from axis 12. An optical beam 9 is spread by the lens so as to illuminate a large area of the filter 4. Since the filter blocks wavelength 1.sub.1, the light is reflected as a beam 11 to a point that is the predetermined distance from axis 12, but on a side of the axis opposite from the point at which the light is injected into optical fiber 10. Accordingly, the light enters optical fiber 14 for transmission. In a similar manner, light of wavelength 1.sub.2 is emitted from source 26 and is injected into lens 8 by optical fiber 16. The light is spread by lens 8 in a beam 13, is passed by filter 4 and is focused by lens 6 onto the end of fiber 14 for transmission on fiber 14.
In the case of a unidirectional receiver, light of both wavelengths is received on optical fiber 14, with the light of wavelength 1.sub.1 being reflected by filter 4 back to optical fiber 10, which light is received by element 18 which in this case would be an optical detector. Light of wavelength 1.sub.2 is passed by filter 4, to be ultimately received by element 26, which would also be a detector.
In the bidirectional case, the assembly can be designed to transmit light of one wavelength and receive light of another wavelength, as, for example, element 18 could be a light source for transmitting light through optical fiber 14, which element 26 would be a detector for receiving light received from optical fiber 14, and vice versa.
In most instances the components of the multiplexer shown in FIG. 1 would be disposed within a housing, with only optical fiber 14 and electrical leads 22 and 30 extending therefrom. However, due to fiber connections between the electro/optical conversion elements and the wavelength division multiplexer, these devices were subject to damage by the surrounding environment. The interconnecting fibers were subject to a high risk of breaking or experienced significant increases in attenuation due to flexure and bending caused by the surrounding environment.
The discrete components required considerable space, resulting in an unacceptably large assembly. If the components were placed in a small housing, the fibers ere subjected to small radius bends, resulting in unacceptable losses.
Operationally, the multiplexer was also deficient due to the need for the light to pass through intermediate fibers represented by fibers 10, 16, 20 and 28 shown in FIG. 1. Losses were introduced by coupling the elements 18 and 26 to pigtails 20 and 28, by splicing fibers 10 and 20, and 16 and 28 by coupling fibers 10 and 16 to lenses 6 and 8 respectively, and by being multiplexed inside the wavelength division multiplexer.
Thus, the prior art devices were unacceptably large, experienced considerable power losses, and were subject to a high risk of breaking or being damaged by the surrounding environment.